CD172a, also known as SIRP alpha (signal-regulatory protein alpha), is a type I transmembrane glycoprotein belonging to the immunoglobulin superfamily. It is expressed on cells of myeloid origin, including monocytes, macrophages, dendritic cells, and granulocytes, but not on T and B lymphocytes. SIRP alpha plays a crucial role in immune regulation by interacting with its ligand, CD47, which is widely expressed on various cell types, including cancer cells. This interaction transmits a "do not eat me" signal to prevent phagocytosis of healthy cells and is involved in processes like dendritic cell-mediated T cell activation and neutrophil migration. Additionally, SIRP alpha has regulatory effects on cellular responses induced by growth factors, insulin, and oncogenes, and it is implicated in both physiological and pathological processes, such as cancer progression.